Stainless steel



Patented Dec. 30, 1941 STAINLESS STEEL Albert L. Kaye, Hammond, Ind., and Robert S. Williams, Belmont, and John Wulif, Cambridge, Mass, assignors to The Chemical Foundation, Incorporated, a. corporation of Delaware No Drawing. Original application April 20, 1937, Serial No. 137,930. Divided and this applica tion July 20, 1939, Serial No. 285,522

1 Claim.

This invention relates to the production of steels, more particularly to stainless steels of the 18-8 type, which are characterized by a high resistance to certain types of corrosion.

The invention also relates to the fabrication of improved steel articles having a special surface, by reason of which the resistance of the article to certain corrosive environments is markedly increased.

The present application is a divisional application of prior application Serial No. 137,930, filed April 20, 1937.

It is becoming increasingly apparent to those skilled in the art that the protection of ferrous alloys against corrosion is not a generic problem susceptible to a generic or panacean solution, but

is, in fact, a series of specific, more or less differentiated problems, certain of which require an individual solution. Current advances in the art more and more definitely establish this characteristic specificity of corrosion not only with respect to the particular alloy to be protected but also the particular corrosive environment to be guarded against.

For example, in the prior application referred to, it was pointed out that while chrome-nickel alloy steels, especially stainless steels of the 18-8 type, are highly resistant to atmospheric corrosion, they are, nevertheless, susceptible to a specific form of corrosion engendered by contact of the steel with saline media. This type of corrosion was, for the sake of a term, called pit corrosion.

It was observed, as pointed out in the prior application, that when a sample of stainless steel was exposed to sea water, and a localized section of the sample .was protected from contact with the water, corrosion took plate in a segregated area adjacent the edge of the protected section. In actual use this peculiar corrosion phenomenon is encountered, for example, when a barnacle attaches to a steel surface which is wetted with sea water. In such circumstance a ring of pits, or cavities, may be formed around the edge of the barnacle.

This type of corrosion differs from general surface or oxidation corrosion not only in the character of the action but also in the locale of attack. Unlike surface oxidation of stainless steel, the products of pit corrosion do not have a protective function on the subjacent metal but, on the contrary, to no inconsiderable degree, tend to accelerate further corrosion. Such pit corrosion, in one respect, is similar to intercrystalline corrosion in that the corroded areas are segregated and tend more or less continuously to intrude into the body of the steel; i. e., like intercrystalline corrosion it tends to a progressive sub-surface action. Unlike intercrystalline corrosion, however, the particular portions of the steel which are corroded appear to be established by extraneous factors rather than by the grain structure of the steel itself.

In many cases the pits or cavities which form in the early stages of the action are so small that they are not noticeable upon a casual, visual inspection, but are clearly apparent only upon a microscopic examination. The insidious and hidden character ofthis type of corrosion, coupled with the fact that it eats into the body of the metal, renders it a particularly destructive form and has imposed marked limitations on the potential field of use of stainless steel.

As pointed out in the prior application, a possible explanation of the mechanism of this type of corrosion can be found in assuming the establishment of an oxygen concentration cell, due to the difference in oxygen concentration of fluids in contact with different parts of the metal. it can reasonably be assumed that the oxygen of the air or sea water is to some degree excluded from the covered or protected portions, while it has free access to and contacts the exposed or unprotected portions. In these circumstances the difference in oxygen concentration establishes a potential difierence, thus initiating electrochemical action between the relatively anodic and relatively'cathodic areas. It would appear that as a result of this action chlorides of the components of the alloy are formed. These chlorides,

being soluble in water, pass into solution and are removed, leaving the denuded areas open to further electrolytic attack. As the pit forms and the action progresses the difference in oxygen concentration between the inside of the pit and the main body of the metal may be accentuated,

and the electrolytic action is continued and accelerated. Such action, no doubt, may be aggravated by other factors, such as additional local concentration cells set up by reason of difierential stresses and the like.

It is to be understood that the foregoing sug gestion of the possible mechanism of this type of corrosion is advanced, not necessarily as an accurate and complete theory of the action, but as indicative of the general character of the action that obtains and more particularly as helpful in explaining the form of protection and the role of the protective agents developed according to this invention.

Thus,

In the earlier application it was clearly pointed out that the ravages of this type of corrosion could be minimized or substantially eliminated by adding to the steel ceitain metal components which could form insoluble chlorides. It was then explained that if slich metals were incorporated in the steel insuch a manner that they were homogeneously distributed throughout the mass of the alloy, then; upon subsequent electrolytic action of the typedeflned, an adherent film of insoluble metal chloride would be established in the corrodible areas. This adherent film or case, being insoluble in the corrosive medium, thus would protect the subjacent metal from attack. It was pointed out that to subserve such a protective function the addition metal should be incorporated withinthe grain structure itself and should not merely be included in the grain boundaries. Inclusion of such a metal only in the grain boundaries would not present the protective function desired, since it would leave the crystals or grains of the alloy open to attack: Indeed, inclusion of such a metal solely in the intercrystalline boundaries would likely accentuate the attack, since it would tend to establish local galvanic couples between the granular andintragranular area.

It was therefore explained that the best method of insuring the uniform and intragi'anular dispersion of the insoluble chloride-forming metal was to utilize a metal which formed a solid solution with one of the components of the alloy. According to the prior disclosure, silver admirably fulfills such requirements. Silver forms a solid solution with nickel, itself a major component of the stainless steel. Similarly silver goes into solid solution with manganese, the latter being a desirable component of certain stainless steels. It was explained that the mere addition of silver, or other insoluble chlorideforming metal, in any steel melt would not satisfy the established requirements. For example, silver is substantially insoluble in iron and chromium. Therefore, if silver were added to an iron chromium melt, upon cooling the silver addition would be substantially completely segregated in the grain boundaries.

It was found also that combinations of molybdenum and silver serve very effectively. Molybdenum, like silver, forms a solid solution with components of stainless steel and also forms an insoluble chloride.

According to the disclosure in the earlier case, therefore, a special steel was described which presented the inherent characteristic of autogenous protection against pit corrosion. The protective function or effect was developed upon subjection of the steel to the corrosive environment during use.

It has now been found that improved results are secured by insuring the formation of. a continuous film of the protective agent on the steel during fabrication so that the steel, or article made therefrom, is provided with a continuous protective film before actual use.

Such a method presents many advantages. In the first place, by utilizing this concept, the protective film of silver chloride, for example, may be formed over the entire exposed area of the steel plate or article, thus insuring the protective effect from the very beginning without resorting to the autogenous development of the film on portions of the metal during use. Again, by establishing the continuous adherent coating under conditions which may readily and accurately be controlled, the degree of formation of the chloride; or the depth of the protective case, may

be governed to conform it to the requirements.

of any particular use.

The present improvement of developing the protective surface as a preformed film presents other and marked advantages in respect .to the character of the protective surface. As explained in the earlier application, the essential feature of this novel mode of protection against pit corrosion is the formation of a tightly adherent or bonded film, or case, which is insoluble in the particular environment, 1. e., a relatively active electrolyte. In this earlier application such film was described as comprising a reaction product of a component of the alloy, such as silver, and a component of the corrosive medium, that is, chlorine. .In these circumstances the ultimate cas'e necessarily was silver chloride. By invoking the principles of the present invention, that is to say by subjecting a. stainless steel article to a special treatment to preform the film, specifically different types of protective films may be produced. In other words, with this type of treatment it is not necessary to form only a silver chloride film, but, on the contrary, the steel article may be treated with any reagent which will react with the silver (or molybdenum) component to form compounds therewith which are insoluble in the saline corwith a preformed film, or case, of other insoluble silver compounds, such as silver cyanide, silver ferricyanide, silver phosphates; and the like. Similarly, where molybdenum is employed as the potential insoluble salt-forming metal, either alone or in conjunction with silver, the fabricated steel article may be subjected to treatment with a suitable reagent to preform, on such article, a continuous adherent film of any suitable insoluble molybdenum compound.

With this concept in view, as will be under-' stood by those skilled in the art, a munber of specifically different types of treating agents may be employed. Such reagents, as the exigencies of a particular case require. may be utilizedin the liquid or vapor phase and under conditions which tend to accelerate the reaction, such as elevated temperatures, pressures, and the like. Similarly, the particular film which is to be formed may be chosen from the relatively large number available to more particularly adapt the treated article to the special use to which it is to be put.

In carrying out the invention it is to be understood that the special l8-8 type stainless steels, described in the earlier application, are preferred. The addition of silver, as there .explained, not only functions ultimately to form a protective surface on the steel but also imparts improved physical characteristics to the steel. such as: increased thermal conductivity, diminished work hardening. improved machinability. and the like. However, it is to be understood that the present invention is likewise applicable to the nickel-chrome alloys generally which are susceptible to pit corrosion, or similar corrosive action, and which are desired to be protected.

With the given concept in mind, a number of methods of establishing a preformed film of an insoluble silver or molybdenum compound will 'chloric acid per liter.

occur to those skilled in the art. For example, as typifying the general method of procedure contemplated, a steel article fabricated from the improved silver-containing stainless steel may be chloridized in any suitable manner to convert the silver in the surface of the alloy to the silver chloride. Thus the fabricated article may first be cleaned, if necessary. by a dip in a relatively dilute sulphuric acid solution. After cleaning, the article may be rinsed and then subjected to the action of a reagent which is effective to form the silver chloride. For this purpose the article may be immersed in an aqueous solution of a suitable metal chloride, a hypochlorite, or the.

like; as a typical example, the article may be immersed for a period of four hours more'or less in an aqueous solution of ferric chloride containing approximately 20 cc. of 2.4 N hydro- The chloridizing reaction may be accelerated by employing elevated temperatures, electrochemical action, and the like. The time of treatment may be prolonged to any desired degree to secure a corresponding depth of the protective silver chloride case.

Again, such chloridizing treatment may be carried out in a suitable apparatus in which the articles are contacted with chlorine gas for a period of from one-half to threeor more hours. Here, as will be understood, the temperature, pressure and moisture condition of the gas, and the time of treatment, may be controlled to secure the desired characteristics in the final film.

It will likewise be appreciated that by utilizing equivalent methods a preformed protective case, or film, of other insoluble silver compounds may be produced. Thus, when desired, the steel articles may be subjected to special solutions to insure the formation of a skin, or film, of insoluble silver phosphates by methods well known to those skilled in the art. Similarly, the articles to be protected may be submitted to any effective cyaniding treatment to form a film of insoluble silver cyanide.

While particular reference is made to treatments adapted to convert the silver component, contained in the surface of the alloy, to insoluble silver compounds, it will be understood that the invention likewise contemplates corresponding treatments for the conversion of molybdenum to insoluble salts, when this particular metal is utilized in the alloy as the potentially reactive protective agent. It will thus be appreciated that the concept of protecting alloys, of the general types described, against this special form of corrosion by establishing a preformed'film presents marked advanta'ges. The protective film may be produced under the mostadvantageous, accurately controlled conditions, with corresponding. improve-' ments in its effectiveness, rather than relying upon the development of such a film during use under uncertain and indeterminable conditions. This method of preforming the protective surface also permits a wide range in the choice of particular compounds utilized; thus at once presenting the possibility of choosing the best surface for a specific or particular service which the article is to undergo. Under the invention, therefore, stainless steel articles of a novel character may be produced, such articles being characterized by an ability to resist the peculiar cor-' rosive efiectof saline media from the outset of service.

Under the invention, therefore, novel types of stainless steel articles may be produced. Such articles may be of any desired shape and size, since the simple types of treatment impose no substantial limitations upon the class of articles that can be processed under the invention.

While preferred modifications of the invention have been described, it is to be understood that these are given to explain the underlying principle, and to typify any method of protecting alloy steels against the type of corrosion described by instituting or establishing a preformed protective film.

We claim:

An improved stainless steel of the low carbon 18 chromium-8 nickel type characterized by a marked resistance to pit corrosion and containing between substantially 16% and 20% of chromium,

between substantially 8% and 12% of nickel molybdenum in effective amounts and up to.

substantially 4%, silver in effective amounts and up to substantially 1%, the remainder being iron, the molybdenum and silver acting to impart to the steel a marked resistance to pit corrosion.

ALBERT L. KAYE. ROBERT S. WILLIAMS I JOHN WULFF. 

